This invention pertains to audio squelch circuits of the type used in radio frequency receivers and, more particularly, to a squelch circuit that adapts its operating parameters as a function of the signal strength of the received radio frequency signal.
In the absence of a radio frequency carrier, a radio receiver will receive and amplify the background noise on the channel, resulting in an objectionable sound at the receiver's speaker. Therefore, in the reception of voice transmissions, radio receivers are usually equipped with a "squelch circuit" to disable the receiver's audio amplifier at the end of a transmission. Since the squelch and audio amplifier circuits do not respond instantaneously, there will be a brief period of time between the end of the transmission and the deactivation of the audio amplifier. During this period of time, which is commonly referred to as "squelch tail", the audio amplifier is active, and the user hears a short burst of noise at the end of every transmission.
A prior art squelch circuit typically deactivates or "squelches" the audio amplifier when the strength of a received radio signal drops below a predetermined threshold voltage, and reactivates or "unsquelches" the audio amplifier when the signal strength exceeds this predetermined threshold. Unfortunately, if the received signal momentarily fades, the audio amplifier may be prematurely disabled. When the signal strength increases after the momentary fade, a significant portion of the transmission may be missed because it requires a finite period of time to detect an increase in signal strength and to reactivate the audio amplifier.
Accordingly, the invention described below maintains the audio amplifier active for a variable period of time after the strength of the received signal drops below a predetermined threshold. This variable time is a function of the strength of the received signal immediately before it fades; the lower the signal strength, the longer the period of time the audio amplifier is activated. The invention also dynamically adjusts the threshold level at which the signal strength must drop below before the audio amplifier can be disabled.